1. Field of the Invention
The invention relates to a method and an apparatus for the detection of contaminants, especially foreign fibers, in an elongated textile product, such as a yarn or a thread.
Before spinning, raw fibers, e.g. cotton fibers, are mechanically cleaned by carding. In this way larger contaminants can be removed. Still, it cannot be ruled out that smaller contaminants, such as foreign fibers, remain in the yarn. These are spun into the yarn and can affect the following processing steps as well as the quality of the final product.
2. Description of the Prior Art
Presently, so-called yarn cleaners are used for controlling the diameter of the yarn and for removing sections of yarn with too large or too small a diameter.
Furthermore, devices have been developed for optically detecting contaminants and removing the corresponding sections of yarn. Such devices, as they are e.g. described in EP-0 197 763 or in CH-674 379, are essentially based on a detection of light reflected from the yarn. Devices based on this optical method recognize the contaminants because of their different optical properties.
However, such optical sensors measuring in reflection, transmission or projection usually generate signals that depend on the diameter of the yarn. Therefore, a basic problem in designing the above mentioned devices lies in generating a signal that is essentially independent of this diameter such that only signals caused by contaminants are detected. The resulting devices are therefore usually complicated and require a careful calibration for each type of yarn, and are difficult to obtain a signal from that is sufficiently independent of the yarn's thickness.